Ok, so it only happens on the last round in the magazine? What are you using for mags? Does it happen on all of them? I'm no pro, but some food for thought : Milsurp and 7.62 ammo, even lighter rounds may not have enough to cycle the gun properly, cheap and easy experiment is try some 168 gr American made .308 rounds. It's new, run her sloppy wet for break in, probably not the root cause though, but will help Maybe you need a heavy buffer to slow down the cylic rate, although since its only on the last round maybe not. There will be more help here soon. Try ammo first.